Thrombotic thrombocytopenic purpura, a fatal disease characterized with disseminated platelet thrombosis in arterioles and capillaries, has been associated with HIV infection. Although plasma exchange or infusion is highly effective and has been the standard treatment since late 1970's, the incidence and case mortality rate of TTP increased by 16 and 3 folds respectively from 1971 to 1991. Concern has been raised that if the trend continues, TTP may become a serious public health problem. The etiologies and pathogenesis of TTP remain uncertain. Abundance of vWF in TTP lesions, as well as frequent occurrence of vWF multimer abnormalities, suggests that TTP is associated with a defect in the regulation of the size of vWF. In normal circulation, vWF, upon release from endothelial cells, is cleaved by a novel plasma metalloproteinase to become smaller forms. Our studies have demonstrated that vWF, when exposed to shear stress in the absence of the proteinase, exhibits an increase in its adhesive activity. Thus, vWF-cleaving proteinase may play a critical role in the prevention of platelet thrombosis. The aim of this project is to determine whether a deficiency in vWF proteinase is involved in the pathogenesis of platelet thrombosis in HIV-related as well as in idiopathic TTP. In a preliminary study of 39 samples from 37 patients with acute episodes of TTP, two of whom were seropositive for HIV, a severe deficiency in the vWF-cleaving proteinase activity was detected in 100 percent of the cases. Inhibitory IgG antibodies to the proteinase were found in at least two third of the TTP plasmas. The deficiency was highly specific since it was not detected in 16 plasmas obtained at remission of TTP or 74 plasmas obtained from normal subjects as well as patients with thrombocytopenia, hemolysis, or thrombosis due to other causes. This project will expand the study on the prevalence and causes of the vWF proteinase deficiency in HIV-related and idiopathic TTP and other disorders with thrombocytopenia and microangiopathic hemolysis, will isolate and clone the vWF proteinase, and will explore, in in vitro and in vivo models, how a deficiency in vWF-cleaving metalloproteinase may lead to platelet thrombosis as encountered in TTP.